Treatment modalities such as Intensity Modulated Radiotherapy (IMRT), helical tomotherapy and radiosurgery are pushing current dosimeter technologies to their limits. Two major driving forces are involved: 1) complex, two-dimensional field patterns encountered in IMRT and tomotherapy require high resolution (i.e., small volume) dosimeters that can be stacked or arrayed to provide a rapid but precise two- or three-dimensional dose measurement, which in turn requires that the dosimeters be water-equivalent in order not to disturb the fluence in the measurement plane and 2) small fields used in all three treatment modalities, down to 1×1 cm2 in IMRT and just a few millimeters in radiosurgery, give rise to volume effects such as spatial averaging in most dose detectors with detecting volumes larger than a few cubic millimeters.
Radiographic films possess high spatial resolution and are used for two-dimensional dose measurements. They are, however, subject to drawbacks. The need to develop the films before reading makes their use for online assessment impossible. Moreover, the development process affects the film response. Radiographic films are also notorious for over-responding to low-energy photons and they are not water-equivalent. Finally, the precision of a radiographic film used for dose measurement in the clinic is often limited to ±5%. Radiochromic films, which do not require development and are closer to water-equivalence in the megavoltage energy range, can also be used to evaluate dose distributions. However, radiochromic films are temperature dependent and sensitive to ultraviolet light. Achieving better than 5% reproducibility in routine fashion with radiochromic film is also challenging.
Detector arrays have been implemented in the clinic to achieve a faster and more precise dose reading than films. To date these arrays have been made of either semiconductor dose detectors or ion chambers. These arrays allow online evaluation of a dose pattern with the precision of a single dosimeter. The spacing between the detectors determines the resolution of an array: the closer the detectors are to each other, the more continuous the dose information will be. Because the materials used with current detector arrays are not water-equivalent (typically made of silicon or air), the use of such an array creates a perturbation in the particle fluence. Moreover, the non-water equivalence of these detectors prevents the use of three-dimensional arrays with closely packed detectors. The current detector arrays also suffer from other limitations depending on the type of detector in use. For semiconductor detectors, there may be significant angular dependence and poor reproducibility for low dose fractions. For ion chambers there may be some dose averaging because their detecting volume is usually larger than other dose detectors. Thermoluminescent detectors (TLD) have also been used in groups to measure dose at different location simultaneously, but the need to read each one individually limits frequent use of a large number of TLDs.
All of the above-mentioned detectors allow, at most, measurements in two-dimensional planes. The only detectors that can be used for three-dimensional measurements are dosimetric gels. Dosimetric gels are either based on the behavior of ferrous ions or on the polymerization of a monomer. They can be produced using a large variety of chemical formulas and each has its own set of advantages and disadvantages. However, most gels share a delicate fabrication process and require a time-consuming development process that makes them unsuitable for online measurements.
One of the key properties that is desired in the next-generation dosimetry systems is water-equivalence. Water-equivalence guarantees that the measurement instrument, when immersed in a water tank, does not perturb the beam fluence and allows stacking and arraying of multiple dosimeters in the treatment field for two- or three-dimensional dose measurements. In order to facilitate the quality assurance of complex treatment modalities, there exists a need for small, water-equivalent dosimeters.